Battery charge is best checked by a load test. Many auto parts stores provide this service for free. That’s the first place to start. I’m assuming the engine isn’t turning over. For the engine to turn over two things have to be verified by measurement:
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Battery must provide at least 9.6 v to the starter battery terminal when the key is in “start”.
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The starter selenoid must receive the “start” signal of specified voltage for the starter, usually at least 10 volts.
That’s usually where a mechanic will start, looking at the voltages at the two starter motor terminals. If the problem is due to a low voltage on the battery terminal at the starter motor when the key is in “start”, that is usually caused by a bad battery or bad battery connections. If the problem is due to not receiving the “start” signal at the SM selenoid, that is usually caused by a bad ignition switch, clutch switch (if manual), neutral saftey switch (if automatic), and/or starter relay.